Injection molding systems have been developed having flow control mechanisms that control the movement of a valve pin over the course of an injection cycle to cause the pin to move either upstream or downstream over the course of injection cycle in order to raise or lower the rate of flow of fluid material to correspond to a predetermined profile of fluid flow rates for the injection cycle. A sensor senses a condition of the fluid material or of the apparatus such as pin position and sends a signal indicative of the sensed condition to a program contained in a controller that uses the signal as a variable input to control movement of the valve pin in accordance with the predetermined profile.